The present invention relates to the establishment of a uniformly distributed electric contact along a joint between two metal surfaces.
When establishing an electric contact between different metal surfaces, there is often a need to obtain uniform distribution of the electrical conductivity of the intended contact surface, and to ensure that this distribution will remain constant during the lifetime of the equipment concerned. This need is particularly manifest in the screening of electromagnetic radiation in such apparatus as computers, radios and radar equipment, for instance. In these contexts, the screening devices often have the form of lidded metal boxes which enclose the electronics and short circuit the electromagnetic fields so as to avoid interaction as a result of electromagnetic induction. At high frequencies, the electromagnetic radiation can be conducted very easily through joints in the metal boxes, resulting in poor attenuation. In order to prevent this, it is necessary to dispose and distribute the electric contact along the joints in a manner to short-circuit the field along the joints. The problem is that electric contact often occurs only at certain points along the length of the joint. It is often at these points that screws and other fastener devices exert a concentrated pressure so that microscopic parts of the surface cut through the oxide layer and contaminants and create a direct metallic connection. At high frequencies, it is necessary to dispose these contact points at a given density along the joints, so as to obtain good attenuation. The distance between each contact point must be made progressively shorter with progressively higher radiation frequencies, which can be calculated by known radiation physical principles. Uniform distribution of the electrical conductivity is often achieved with the aid of packings comprised of felted metal fibres or metal foil strips which have been embossed in a manner such as to obtain resilient parts repeatedly along the length of the joint. Rubber packings which include metal or carbon fillers are also used with the intention of combining an elastic and electrically conducting packing function. Despite these arrangements, it is often difficult to maintain an effective screening function under different circumstances and during the useful life of the equipment.
The positioning of the screws along the joint and the evenness with which the screws are tightened is highly significant to the screening effect obtained. The material thickness of the lid, for instance, is also highly significant in maintaining joint pressure, which is one prerequisite of a uniformly distributed contact and therewith screening. Materials which form oxidized surface layers are highly disadvantageous, since conditions under which these surface layers can be penetrated are lacking. Penetration of such surfaces would require a much higher joint pressure or require the joint surfaces to comprise noble metals, such as gold, copper or silver. From a constructional and technical aspect, those metals which do not easily oxidize present a serious drawback because they are both heavy and expensive. Plating or cladding of aluminium alloys, for instance, is both complicated and subjected to quality problems. If aluminium is to be contacted electrically, it is necessary to penetrate the very hard aluminium-oxide surface of the metal, which requires the application of a very high punctiform pressure. If a large number of such points or sites are to be contacted electrically, the combined joint pressure will be very high. When applying established methods, a serious problem arises in obtaining such contact points simultaneously. Added to this is the desire to use inexpensive, lightweight materials and only a few fastener devices, for both technical and economical reasons.